1. Field of the Invention
The present invention relates to a power supply device used in an image forming apparatus. More particular, the invention relates to a power supply device used in an inductive-heating-type heating unit, the heating unit that uses this power supply device, and an image forming apparatus that uses the heating unit.
2. Description of the Related Art
An electrophotographic image forming apparatus is equipped with a fixing device for fixing a toner image that has been transferred to a print medium such as paper. Although devices using a ceramic heater or halogen heater are in wide use, such fixing devices that rely upon electromagnetic inductive superheating have come into use in recent years. A voltage-resonance-type power supply shown in FIG. 10 (see the specification of U.S. Pat. No. 6,608,289, for example) is an example of a power supply device for supplying power to a fixing device that uses inductive heating.
A power supply device 524 shown in FIG. 10 includes a bridge diode D101, a smoothing capacitor C210, a resonant capacitor C201, a switching element SW401, a driving circuit 410 for driving the switching element SW401, and a control circuit 411 for controlling the driving circuit 410. The power supply device 524 is adapted so as to receive commercial power P510 as an input and pass a high-frequency current through a coil 71. In this arrangement, output power is controlled by changing the switching frequency of the switching element SW401. So-called “soft switching” is carried out by causing the coil 71 and resonant capacitor C201 to resonate and performing switching when the voltage impressed upon the switching element SW401 becomes zero, thereby diminishing switching loss.
FIG. 11 illustrates emitter-collector voltage Vce of the switching element SW401 in power supply device 524, current Ic that flows into the switching element SW401, loss P produced in the switching element SW401, and driving signal CS401 of the switching element SW401.
The switching element SW401 is controlled so as to be turned on and off when at the high and low levels, respectively, and passes a high-frequency current through the coil 71 by being turned on and off. The coil 71 is connected in parallel with the resonant capacitor C201 and an AC voltage is applied thereto by turning the switching element SW401 on and off. The coil 71 and resonant capacitor C201 form a parallel resonant circuit and switching of the switching element SW401 is performed at a frequency lower than the resonance frequency. The closer the switching frequency is to the resonance frequency, the higher the impedance of the parallel resonant circuit and the more difficult it becomes for current to flow. The closer the switching frequency is to the resonance frequency, therefore, the lower the power that is input to the coil 71. At the time of a low power output, therefore, the switching frequency is raised and approaches the resonance frequency. However, if the switching frequency comes too close to the resonance frequency, the resonating state can no longer be maintained. As a consequence, the switching element turns on without a sufficient decline in the voltage impressed upon the switching element, and there is an increase in the proportion of loss.
At the time of low power output, therefore, the switching element is driven at a switching frequency within a range that will not deviate from resonance and the switching operation is changed over from continuous to intermittent operation to thereby realize a low power output efficiently.
In the above-mentioned voltage-resonance power supply device, an IGBT (insulated-gate bipolar transistor) is often used as a switching element for dealing with high power. As far as the element characteristics of an IGBT are concerned, there is a so-called “tail phenomenon” in which current flows for a fixed period of time even after the element is switched off. A current Ic[A] that flows into the switching element SW401 has the waveform shown in FIG. 11. Specifically, at timing t1 in FIG. 11, the switching element SW401 is turned off and the current Ic[A] that flows into the switching element SW401 decreases. In an interval up to t2, however, the current increases again and then decreases again. This is a tail current It. In the interval from t1 to t2, a loss Pt[W] occurs and is ascribable to the tail current It and a voltage Vce that rises from timing t1. Owing to the tail phenomenon, loss is produced by the rise in voltage and tail current after a switch-off even though the voltage is applied to the switching element at the time of switch-off.
The fact that the proportion of noise due to the tail phenomenon is a major part of the loss produced by the overall power supply circuit is a problem. The method described in the specification of U.S. Pat. No. 6,608,289 seeks to improve upon efficiency at the time of low-power output. However, the method is not effective in reducing loss and improving efficiency at the time of ordinary operation, namely at the time of operation other than low-power operation, and there is no reduction in loss ascribable to the tail phenomenon.